1. Field of the Invention
The present invention relates to a liquid crystal display apparatus with a wide viewing angle and a method for producing the same.
2. Description of the Related Art
In a liquid crystal display (LCD), a liquid crystal layer including liquid crystal molecules is provided between a pair of substrates. When the orientation direction of the liquid crystal molecules is changed, the birefringence of the liquid crystal layer is also changed. By utilizing the change in the refractive index, the LCD performs the display. Accordingly, it is important that the liquid crystal molecules are arranged as regularly as possible in the initial state. In order to regularly arrange the liquid crystal molecules in the initial state, the surface conditions of the substrates which sandwich the liquid crystal layer should regulate the interactions between the liquid crystal molecules and the surfaces.
In the method for performing such a regulation which is currently the most widely used, material for a liquid crystal alignment film is applied to each of the surfaces of the substrates which face the liquid crystal layer. The applied material is dried and cured, so as to form the alignment film. Thereafter, the surface of the alignment film is rubbed. Thus, the liquid crystal molecules can be aligned in the rubbing direction. The rubbing treatment is unidirectionally performed on the entire substrate, so that the liquid crystal molecules in the liquid crystal layer which are in the vicinity of the substrate surface are aligned in one direction. In addition, the tilt angles (i.e., pretilt angles) of the liquid crystal molecules in the vicinity of the substrate with respect to the substrate surface are substantially equal to each other.
In an LCD which uses thin film transistors (TFTs) as switching elements, i.e., in a TFT-LCD, the construction of a twisted nematic (TN) type liquid crystal layer is adopted (an LCD of the TN mode). In such an LCD of the TN mode, the liquid crystal molecules between the pair of substrates are continuously twisted by 90.degree. along the direction perpendicular to the surfaces of the substrates, by means of the alignment films formed in the inward-facing surfaces of the substrates.
FIG. 18 is a plan view of an exemplary TN type LCD, and FIG. 19A shows a cross section of a picture element portion of the TN type LCD. The LCD is a TFT-LCD of an active matrix type. As is shown in FIG. 19A, a liquid crystal layer 133 is sandwiched between substrates 131 and 132 which are provided so as to face each other. The substrates 131 includes a glass substrate 131a on which scanning lines 112 and signal lines 113 are formed so as to cross each other as shown in FIG. 18. In the vicinity of the crossings of the scanning lines 112 and the signal lines 113, thin film transistors (TFTs) 120 as nonlinear switching elements are formed. In areas enclosed by the scanning lines 112 and the signal lines 113, pixel electrodes 110 are formed, respectively, in such a manner that part of each pixel electrode 110 and the scanning line 112 are overlapped. The area 118 in which the pixel electrode 110 and the scanning line 112 are overlapped functions as an additional capacitance. Each of the TFTs 120 includes a gate electrode 115 which is branched from a scanning line 112, a source electrode 116 which is branched from a signal line 113, and a drain electrode 117 for connecting the TFT 120 to a pixel electrode 110. Over the glass substrate 131a on which the above-mentioned elements are formed, an insulating protective film 131d and an alignment film 131e are formed in this order.
The other substrate 132 also has a glass substrate 132a on which a color filter 132b and a transparent electrode 132c are formed in this order. Over the glass substrate 132a on which the above-mentioned elements are formed, an insulating protective film (not shown) and an alignment film 132e are formed in this order. The alignment film can also function as the insulating protective film.
In the liquid crystal layer 133 sandwiched between the above-described substrates 131 and 132, the liquid crystal molecules are aligned so that the orientation directions are continuously twisted by 90.degree. along the direction perpendicular to the surfaces of the substrates. A liquid crystal molecule 133a near the center position along the direction perpendicular to the surfaces of the substrates has a predetermined angle with respect to the substrate surface. The substrates 131 and 132 are sealed at their ends by a resin or the like (not shown), and a peripheral circuit or the like for driving the liquid crystal is externally mounted. LCDs which are of types other than the active matrix type have the same construction as that described above.
In the TN type LCD, by the application of a voltage across the substrates 131 and 132, an electric field is generated in a direction perpendicular to the surfaces of the substrates 131 and 132. In accordance with the dielectric anisotropy of liquid crystal, the liquid crystal molecules stand. By aligning the liquid crystal molecules in parallel to the direction of the electric field, the birefringence of the liquid crystal layer 133 is varied. If the direction of the electric field is perpendicular to the direction to which the liquid crystal molecules stand during no voltage application, that is, if the pretilt angle is 0, the direction to which the liquid crystal molecules stand is not uniquely determined. As a result, a disclination line is generated between liquid crystal domains having different standing directions in response to the electric field. Such a disclination line degrades the display quality. Thus, in order to prevent the generation of the disclination line, as shown in FIG. 19A, the liquid crystal molecules are previously set to be tilted (i.e., to have a pretilt angle).
FIG. 19B shows the initial orientation of the liquid crystal when the liquid crystal panel shown in FIG. 19A is viewed from the side of the substrate 132 which is the upper one in FIG. 19A. Vector a in FIG. 19B indicates the rubbing direction of the alignment film 132e, vector b indicates the rubbing direction of the alignment film 131e. The liquid crystal molecules in the vicinity of each of the alignment films 131e and 132e are aligned along the respective rubbing direction (a or b in FIG. 19B) with a pretilt angle 6. The rubbing directions a and b forms an angle of 90.degree. therebetween (the twist angle .theta.t=90.degree. in FIG. 19B). The liquid crystal molecules in the liquid crystal layer 133 are continuously twisted by 90.degree. along the thickness direction of the liquid crystal layer 133. Accordingly, the liquid crystal molecule 133a near the middle position in the thickness direction of the liquid crystal layer 133 is also tilted by the angle .delta. with respect to the substrates 131 and 132. The orientation direction of the liquid crystal molecule 133a near the middle position is indicated by vector c in FIG. 19B. The vector c divides the twist angle .theta.t into two equal angles.
Herein, the plus side of the viewing angle .theta.v in FIG. 19A (the side indicated by .theta.1) is referred to as a positive viewing direction, and the minus side of the viewing angle .theta.v in FIG. 19A (the side indicated by .theta.2) is referred to as a negative viewing direction. Specifically, the direction in which the liquid crystal panel is viewed from the viewing point on the right side of the vertical broken line in FIG. 19B (i.e., the line which is perpendicular to the orientation direction c of the liquid crystal molecule near the center position of the liquid crystal layer, and which divides the liquid crystal panel into two equal parts) is referred to as the positive viewing direction. The in-plane orientation direction of the liquid crystal panel of the liquid crystal molecule 133a positioned near the center of the liquid crystal layer (c in FIG. 19B) is referred to as a reference orientation direction. As is seen from FIG. 19B, the reference orientation direction divides the twist angle .theta.t of the liquid crystal layer 133 into two equal angles. Also, the minus direction of c is referred to as a reference viewing direction v. That is, the reference viewing direction v is the representative positive viewing direction.
Also herein, an imaginary clockface ( dial ) is drawn on the liquid crystal panel, and the orientation direction of liquid crystal in the liquid crystal layer is indicated by the clock representation method. Specifically, in the construction in which the display on the liquid crystal panel is actually viewed by a viewer, the upper side of the liquid crystal panel is represented as 12 o'clock, the lower side thereof is represented as 6 o'clock. In a similar way, the orientation direction of the liquid crystal layer is represented as the time in the imaginary clock indicated by the reference orientation direction of the liquid crystal layer in the liquid crystal panel. For example, the liquid crystal layer having the reference orientation direction c as shown in FIG. 19B is represented in such a manner as to "be oriented at 3 o'clock" in the construction in which the front side of the figure sheet is regarded as the upper side of the liquid crystal panel.
The TN mode LCD, since the liquid crystal molecules are aligned in the above-described manner, there occurs a phenomenon in which the contrast is different depending on the angle at which the LCD is viewed. The reasons why the contrast changes will be described below.
FIG. 20 shows an exemplary applied voltages to transmittance characteristics in a normally white mode of an LCD in which light is transmitted during the no voltage application so as to perform a white display.
In FIG. 20, the solid line L1 shows the applied voltage to transmittance characteristic when the LCD shown in FIG. 19A is viewed in the direction perpendicular to the surfaces of the substrates (.theta.v=0.degree.). In this case, as the applied voltage value becomes high, the transmittance of light is decreased. When the voltage value reaches a specific value, the transmittance becomes substantially equal to zero. Accordingly, even when a much higher voltage is applied, the transmittance remains at substantially zero.
When the viewing angle is inclined from the direction perpendicular to the substrate face to the positive viewing direction, the applied voltage to transmittance characteristic is varied as is shown by solid line L2 in FIG. 20. Specifically, as the applied voltage becomes higher, the transmittance is decreased to some extent. When the applied voltage exceeds a specific value, the transmittance is increased. Then, the transmittance is gradually decreased. Therefore, when the viewing angle is inclined in the positive viewing direction, there occurs a phenomenon in that the black and the white (the negative and positive) of the image are inverted at a specific angle. This phenomenon occurs because the apparent birefringence of liquid crystal molecules having optical anisotropy is varied depending on the viewing angle.
Referring to FIGS. 21A to 21C, the phenomenon will be described in detail. As is shown in FIG. 21A, when the applied voltage is zero or a relatively lower voltage, the center molecule 133a of the liquid crystal layer is observed in the form of an ellipse by the viewer 137 positioned in the positive viewing direction. As the applied voltage is gradually increased, the center molecule 133a is moved in such a manner that the longer axis becomes aligned along the direction of the electric field, i.e., the direction perpendicular to the substrate face. Accordingly, the center molecule 133a is momentarily observed in the form of a circle by the viewer 137, as is shown in FIG. 21B. As the voltage is further increased, the center molecule 133a becomes substantially in parallel to the electric field direction. As a result, the center molecule 133a is observed again in the form of an ellipse by the viewer 137, as is shown in FIG. 21C. In this way, the inversion phenomenon occurs.
If the viewing angle is tilted in the negative viewing direction, the variation of the light transmittance with respect to the applied voltage is relatively small as compared with the case of being viewed from the direction perpendicular to the substrates, as is shown by solid line L3 in FIG. 20. As a result, when the LCD is viewed from the negative viewing direction, the inversion phenomenon does not occur, but the contrast is greatly degraded.
In the TN mode LCD, the inversion phenomenon when viewed from the positive viewing direction and the degradation of contrast when viewed from the negative viewing direction cause serious problems for the viewer, and they result in doubts about the display properties of the LCD.
As to techniques for improving the inversion phenomenon, JAPAN DISPLAY '92, pages 591-594, and page 886 describe the following two methods. In one method, the surface of the alignment film is unidirectionally rubbed, and then a resist is deposited on a part of the alignment film. Then, the rubbing is performed to the portion which is not covered with the resist in the direction reversed from the previous rubbing direction. Thereafter, the resist is removed. As a result, one and the same liquid crystal cell includes a different orientation direction of liquid crystal molecules in the vicinity of the center of the liquid crystal layer. In the other method, polyimide alignment films made of different compositions are juxtaposed and then they are subjected to the rubbing treatment. As a result, a plurality of pretilt angles are formed depending on the compositions thereof. According to these methods, two types of regions having opposite reference orientation directions are formed in one and the same cell, so that the viewer can mixedly observe the viewing characteristics in both directions. As a result, the inversion phenomenon when viewed from the positive viewing direction and the significant reduction of the contrast when viewed from the negative viewing direction are reduced and improved.
As described above, the viewing characteristics in the positive viewing direction and in the negative viewing direction are made uniform, but there exists another viewing characteristic in the direction perpendicular to the two reference orientation directions which are opposite to each other by 180.degree. (i.e., in 6 o'clock or 12 o'clock direction when the two reference orientation directions are regarded as 3 o'clock and 9 o'clock direction ). The viewing characteristic in the perpendicular direction is different from the viewing characteristics in the positive and negative viewing directions. The above methods cannot make the entire viewing characteristics uniform. In the above-described methods, regions having different aligning conditions are formed on both alignment films, so that it is necessary to align the boundary between the regions having different aligning conditions on one substrate with the boundary on the other substrate when the substrates are attached to each other. However, it is extremely difficult to precisely align the boundaries with each other in the actual process. Thus, it is necessary to form a light blocking film in view of the possible deviation of the alignment of boundaries. This causes the opening ratio to be decreased.
Display apparatuses have various applications, so it is desired that the screen display can attain equal viewing characteristics for wide angles in all viewing directions. There may be cases where equal viewing characteristics for wide angles are required in three directions (e.g., at 3 o'clock, 6 o'clock, and 9 o'clock directions) or where equal viewing characteristics for wide angles are required in two directions (e.g., at 3 o'clock and 6 o'clock directions). Thus, it is desirable that viewing characteristics required for the desired application can be obtained.
In another attempt to eliminate the inversion phenomenon when viewed from the positive viewing direction and the degradation of contrast when viewed from the negative viewing direction, a rectangular region 119 as shown in FIG. 18 in which the orientation direction of the liquid crystal molecule 133a in the vicinity of the center of the liquid crystal layer 133 is different from that in the other region is formed in the picture element shown by a broken line in FIG. 18. In more detail, two regions having reference orientation directions which are different from each other by 180.degree. are formed in one picture element, so that the contrast degradation when viewed from the negative viewing direction is compensated, and the inversion phenomenon when viewed from the positive viewing direction is suppressed.
However, in the case where one picture element has liquid crystal layer regions having different reference orientation directions formed therein, as the time elapses, the aligning condition of one liquid crystal layer region may be absorbed by the aligning condition of the other liquid crystal layer region. In addition, in the boundary area between the liquid crystal layer regions, a disclination line occurs. This causes the contrast to be degraded.